The salient advantage of wireless telecommunications over wireline telecommunications is that the user of the wireless terminal is afforded the opportunity to use his or her terminal anywhere. On the other hand, the salient disadvantage of wireless telecommunications lies in that fact that because the user is mobile, an interested party might not be able to readily ascertain the location of the user.
Such interested parties might include both the user of the wireless terminal and a remote party. There are a variety of reasons why the user of a wireless terminal might be interested in knowing his or her location. For example, the user might be interested in telling a remote party where he or she is or, alternatively, the user might seek advice in navigation.
In addition, there are a variety of reasons why a remote party might be interested in knowing the location of the user. For example, the recipient of an E 9-1-1 emergency call from a wireless terminal might be interested in knowing the location of the wireless terminal so that emergency services vehicles can be dispatched to that location.
There are many techniques in the prior art for estimating the location of a wireless terminal. In accordance with some techniques, the location of a wireless terminal is estimated, at least in part, from signal measurements that are reported by the wireless terminal. The reported measurements are of signals measured by the wireless terminal that are transmitted by one or more base stations and/or by Global Navigation Satellite System (GNSS) satellites, such as Global Positioning System (GPS) satellites. In order for these techniques to work, at least some of the transmitted signals have to be strong enough to allow for accurate measurement by the wireless terminal and for reliable processing by the particular estimation technique. Some of these techniques work well even in environments where the measured strengths of the different signals vary significantly, such as where signal obstructions are present, including natural obstructions such as mountains and artificial obstructions such as buildings.
There are also techniques in the prior art for estimating the elevation of a wireless terminal. Some of these techniques rely on the relationship between barometric pressure, POBJ, and elevation, ZOBJ, according to the formula:
                              Z          OBJ                =                                            -                              H                OUT                                      ·                          ln              ⁡                              (                                                      P                    OBJ                                                        P                    W                                                  )                                              +                      Z            W                                              (                  Eq          .                                          ⁢          1                )            wherein:                HOUT is the outdoor scale height of the atmosphere, which is the elevation at which the atmospheric pressure has decreased to e−1 times its value at mean sea level (e.g., approximately 8400 meters) and is based on outdoor temperature at a pressure station reference.        POBJ is the relevant measurement of barometric pressure at the elevation of interest,        PW is the measurement of atmospheric pressure at the pressure station reference, and        ZW is the elevation of the pressure station reference.        
It is well known in the art how to estimate the elevation of an object using Equation 1. For example, it is well known in the art how to estimate the elevation of a wireless terminal using Equation 1, in which barometric pressure measurements made by the wireless terminal can be used.
Such estimates of elevation based on barometric pressure, however, can be inaccurate for a variety of reasons. For example, the reference atmospheric pressure PW, which is often provided by a pressure station at a nearby airport, often has a measurement bias error. There are historical reasons for this type of error: until relatively recently, airport pressure stations only reported atmospheric pressure to a resolution of 0.01 inches of mercury (inHg), which corresponds to almost three yards of height. In an aviation context, this level of reporting resolution has been sufficient.
Even though current generation pressure stations are capable of better reporting resolution than what has been required for aviation purposes, measurement bias error is often still present at airport, and other, pressure stations. Although the measurement bias error might be acceptable in an aviation context, the same amount of error is unacceptable in other contexts. A context in which the error is problematic is in an emergency response system, which often needs to ascertain the elevation of a person or object, such as within a building, to within a few feet of distance.
Furthermore, the published reference elevation ZW of an airport pressure station is often unreliable as well and also unacceptable in certain situations that require higher accuracy in the estimation of an object's elevation.
As can be seen in Equation 1, an incorrect value of the reference atmospheric pressure PW or the reference elevation ZW can result in an inaccurate estimate of elevation.